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   Rattus exulans (mammal)
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    Details of this species in New Zealand
    Status: Alien
    Invasiveness: Invasive
    Occurrence: Present/controlled
    Source: Atkinson and Moller, 1990
    Arrival Date: Introduced by Maori settlers in the 10th century.
    Introduction: Intentional
    Species Notes for this Location:
    Pacific rats (Rattus exulans) were introduced to New Zealand perhaps as much as 2000 years ago by Maori, who used them as a food source. A combination of intentional liberations and natural dispersal led to Pacific rats being widely dispersed around New Zealand and on its offshore islands (Thomas and Taylor, 2002). Pacific rats are poor swimmers and have reached New Zealand mainland and offshore islands through deliberate and accidental introduction by humans. They are unique because of their association with the migration of Polynesians throughout the Pacific and because of their cultural and spiritual values to some iwi Maori (DOC). Pacific rats in New Zealand are most closely related to R. exulans from the Southern Cook and Society Islands (Atkinson and Towns, 2001).

    It is thought that there are competitive effects between other rodents in New Zealand (Rattus rattus, R. norvegicus and Mus musculus) and Pacific rats. Coexistence of the four species in the same habitat is yet to be proven in New Zealand, although it is common on warmer islands in the Pacific such as Hawaii (Atkinson and Towns, 2001). A significant range contraction is occurring, probably caused by competitive interactions with R. rattus and to a lesser extent R. norvegicus. Today, the Pacific rat is found only in South Westland and Fiordland on the mainland (Veitch et al., 1992).
    Pacific rat populations probably established in northernmost New Zealand and surrounding islands about 750 BP (Brook, 2000; in Atkinson and Towns, 2001). Today, there impacts on the mainland are primarily of historical significance only, but they remain dominant on some smaller islands where one or more of the other rodent species in New Zealand are absent (Broome, 2004). They are generally found in areas with well-vegetated ground cover and well-drained soil (Harper et al., 2005).

    Holdaway (1999) presents a model for the invasion of New Zealand by Pacific rats.

    Management Notes for this Location:
    The Department of Conservation (New Zealand) has successfully eradicated Pacific rats (and other rodents) from a number of offshore islands and plans to pursue eradication for all fifteen islands under its administration. Various methods have been used to eradicate rodents from offshore islands including trapping, 1080 and talon-type baits. Recent advances, including the development of more effective anticoagulant poisons and new application methods, mean it is now possible to consider rodent eradication operations on larger islands. The Department, health and regional authorities are involved in poisoning operations to ensure risk to humans, the environment and non-target species is minimised (DOC). A strategy for Pacific rats management has been produced (DOC, 1995; in Atkinson and Towns, 2001) which aims to: 1) Eradicate or exclude Pacific rats from islands where there is a need to manage or restore indigenous species or ecosystems, 2.) Further evaluate the impacts of Pacific rats on indigenous flora and fauna, 3.) Consult with tangata whenua, community groups and other interested parties in managing or restoring indigenous species or ecosystems on islands occupied by Pacific rats and 4.) Allow, and where appropriate support, the collection for research purposes of live and/or dead Pacific rats prior to any eradication.

    Towns and Broome (2003) outline the history of rat eradications in New Zealand.

    Location Notes:
    Includes Stewart Island, Chatham Islands (eradicated) and smaller offshore islands.
    Ecosystem change: Possible effects of Pacific rats (Rattus exulans) on selected indigenous tree species in coastal forests of northern New Zealand are surveyed from recent field sampling and a literature review. Recruitment rates are compared on islands with and without Pacific rats: (i) on the same island before or at the time of rat eradication compared with recruitment some years later, and (ii) on geographically separated islands with and without Pacific rats. In addition, Pacific rats-proof exclosures enabled some comparisons to be made of seed germination and survival in the presence and absence of Pacific rats. There is evidence that Pacific rats have substantially reduced recruitment of Pittosporum crassifolium, Pouteria costata, Streblus banksii, and Nestegis apetala, by eating the seed. Seed consumption and/or depressed recruitment is demonstrated for Rhopalostylis sapida, Vitex lucens and Pisonia brunoniana, but the extent of recruitment reduction is not yet clear. No depressive effect by Pacific rats on the recruitment of some species, including Dysoxylum spectabile, Beilschmiedia tawa, B. tarairi, Corynocarpus laevigatus, Melicytus ramiflorus, Pseudopanax arboreus, P. lessonii, and Coprosma macrocarpa, has yet been demonstrated; juveniles remain abundant in the presence of Pacific rats. Some tree species most affected by Pacific rats are now rare in coastal forest of the northern islands and mainland. Evidence from recruitment reduction in these species suggests that the composition of northern coastal forest before Pacific rats arrived was significantly different from that of the present. It also suggests that, if rats are present, current successional pathways following burning or other disturbance of coastal forest will not restore the forest to its pre-human composition (Campbell and Atkinson, 1999).
    Reduction in native biodiversity: Finsch’s duck (Chenonetta finschi), an extinct, possibly flightless New Zealand endemic, was widely distributed and apparently abundant immediately before human settlement of New Zealand, but its bones have rarely been identified in archaeological sites. Its extinction has been variously attributed to habitat changes, predation by the introduced Pacific rat (R. exulans), and human predation (Holdaway et al 2002).
    Last Modified: 18/08/2006 1:10:26 p.m.

ISSG Landcare Research NBII IUCN University of Auckland